Sand is used in metal foundries for casting metal into desired shapes. The sand casting process of shaping metal objects is ancient. The sand used in the sand casting process is selected and treated according to a wide variety of requirements relating to the type of metal and object being cast. Often some of the sand is treated with resins, which along with moisture, provide a consistency that is needed to maintain the requisite shapes within the molds into which the molten metal is poured. The grain sizes porosity, and chemical makeup of the sand are all factors which affect the quality of castings.
Sand may be reused many times in a sand casting foundry. As it is used it becomes progressively contaminated with certain impurities which include constituents of the metal being cast, chemical reaction products of the resin additives, water, air, the cast metal and the sand itself. At some point its characteristics change sufficiently that it must be discarded and replaced with a fresh supply of sand. The discarded sand is a waste product which contains impurities which may include resins, metals, and reaction products of these with environmental elements.
The impurities in waste foundry sand may be leached out into the natural environment where they may be hazardous. Rain and its action together with organic acids natural to the environment may dissolve certain of these impurities and cause them to become contaminants in local water supplies.
In brass and bronze casting foundries, lead becomes a hazardous contaminant in the waste sand since it is a component of many alloys commonly used for casting.
Waste foundry sand at a brass casting foundry, for example, will typically contain approximately 3000 ppm (parts per million, by weight) more or less of lead, of which approximately 600 ppm may be soluble in aqueous solutions of acetic acid. Under conditions designed by the U.S. Environmental Protection Agency (EPA) to simulate the natural environment, the leachate from such waste foundry sand will typically contain approximately 30 ppm, more or less, of lead. The simulation conditions specified by the EPA standard test require leaching with an aqueous acetic acid solution with a pH of 5.0.+-.0.2. This is designed to simulate naturally occuring acidic water which may form from the action of rain and ground water on the environment. Using this standard test, samples of leachate from waste foundry sand at the Phelps Dodge, Lee Brothers Brass Foundry in Anniston, Ala. were found to contain 14,28,33 and 37 ppm, respectively, of lead.
The Environmental Protection Agency designates a waste material as hazardous if it exceeds 5 ppm of acetic acid soluble lead leached by its standard test solution. This is set forth in Federal Regulation CFR 40 Section 132.0102, Mar. 11, 1980.
The invention relates to a method of treating the waste sand so that its acetic acid soluble lead can be reduced to a safe level, below 5 ppm.
Prior efforts to reduce the lead content of waste materials are directed toward removing, separating, or extracting the lead from the material. For example, a process known as the "Lime and Settle Treatment", which removes in excess of 90% of lead, Mercury, and zinc from waste water is utilized in the lead industry.
Another example may be seen in the recovery of lead from lead oxide by reducing lead oxide in the presence of carbon.
The reclamation of foundry sand and recovery of metals from waste foundry sand has been studied by G. V. Sullivan and E. G. Davis for the United States Bureau of Mines. The recovery of copper from brass foundry wastes, chromite from steel foundry sand, and the removal of phenolic resin binders is reported in their reference:
"Development and Economics of Treating a Brass Foundry Waste" by G. V. Sullivan and E. G. Davis PA1 Proceedings of The International Pollution Engineering Congress Paper delivered at McCormick Place, Chicago, IL, September 1974, Session 10, Page 59 PA1 "Phase Diagrams for Ceramists" by Ernest Levin, Howard McMundie, and F. P. Hall Ed. by American Ceramics Society, 1956, Page 59, FIG. 99,
The above reference discusses several techniques of recovering foundry sand and metals contained in it. Froth flotation, electrostatic separation, gravity concentration and standard ore processing techniques are included. The above reference indicates the economic benefits of recycling foundry sand and recovering copper from it.
The removal of hazardous contaminants, such as lead, when they are found in small concentrations in sand may not be economically viable, whereas metals which may appear in higher concentrations, such as copper, may be removed and employed.
The cost of removing hazardous contaminants, such as lead, which are found in small concentrations in sand may be greater than the value of the hazardous substance recovered. This invention is especially useful in this situation, where the sand can be made safe without removing the hazardous substance.
This invention may enable a cost saving to be realized in the disposal of a hazardous waste. The generally higher cost of directly disposing of a hazardous waste may be reduced to the generally lower cost of disposing of a non-hazardous waste after treatment by the subject process.
The combining of certain materials, particularly lead and its oxide forms, into silicon dioxide (silica) and silicates is well known in the ceramic arts. Lead, which is an environmentally hazardous substance, is added to glass in order to alter its properties. The resultant compositions are less soluble in aqueous solutions of acetic acid, and therefore environmentally safe. The addition of lead and lead compounds to glass can be used to change its melting point, index of refraction, coloration, and mechanical strength.
The production and application of glazing compounds makes use of the reduction of melting point caused by the addition of lead and other substances to glass.
The effects of temperature on the properties of various chemical combinations as their proportions are varied is found in:
which shows the melting temperature of a mixture of lead oxide (PbO) and silicon dioxide (SiO.sub.2) in a range from 100% PbO, 0% SiO.sub.2 to 35% PbO, 65% SiO.sub.2. This phase diagram shows that the melting point of PbO-SiO mixtures falls below 800.degree. C. over a wide range of proportions. The melting point of pure PbO and pure SiO.sub.2 is in excess of 800.degree. C.
A process for treating an arsenic-containing waste to produce a landfill material impervious to the leaching effects of ground or rain waters is described in U.S. Pat. No. 4,142,912, issued Mar. 6, 1979, to Dean A. Young. The process described by Young requires materials to be added to the waste material and curing of the waste material into a rock-like aggregate.